Med-Tech Innovation Surface Treatment
hydrophobic and hydrophilic surfaces is contact angle. This is the angle a droplet of fluid, usually water, makes at the point of contact with a rigid solid surface. For hydrophobic coatings, this angle is always greater than 90 degrees, and it can be as high as 150 degrees. Hydrophilic coatings always have contact angles less than 90 degrees and usually less than 50 degrees. A “perfect” hydrophilic surface would give a contact angle of zero degrees, that is, the water would sheet evenly on the surface.
Although both types of coatings have relatively low coefficients of friction compared with common substrates found in medical devices such as polyurethane or nylon, hydrophilic coatings tend to be an order of magnitude more lubricious. Some of the best hydrophobic coatings offer coefficients of friction in the range of approximately 0.15 to 0.3. By contrast, hydrophilic coatings that claim to be exceptionally lubricious have coefficients of friction in the range of 0.005 to 0.2. This advantage comes at a cost, however. Hydrophilic coatings, by their nature, must be wet in order to exhibit lubricity, whereas low friction hydrophobic coatings do not need to be wet. A dry hydrophobic coating is more lubricious than a dry hydrophilic coating in most cases. One area where both types of coating are similar is how they are affected by cross-linking. Generally, there is an inverse relationship between lubricity and degree of cross-linking: the more cross-linked a material is, the less likely it is to be slippery. Cross-linking also affects basic polymeric properties such as glass transition temperature, stiffness and solubility. These factors, together with polar interactions, all relate to how a surface interacts with water because they control how easily a material absorbs or repels water. Physically, materials that are cross-linked also have higher durability.
Often, materials used in medical devices do not
require durability to be as high as materials used in industrial applications. For example, a catheter used for accessing the coronary arteries for stent delivery is a disposable device. A hydrophilic coating on that catheter would typically need to withstand one or two cycles of sterilisation and up to ten passes through the vasculature. The same coating on an outdoor lighting system exposed to sun, rain and temperature fluctuations for permanent duration would fail. A design engineer would choose a hydrophobic coating to withstand those conditions, a higher coefficient of friction not being as relevant.
Applications
In the medical device industry there is considerable overlap between applications that use hydrophobic and hydrophilic coatings. Sometimes, a project will start out using a hydrophobic coating only to find that it is not lubricious enough, and efforts will then focus on switching to a hydrophilic coating. Conversely, a hydrophilic coating may prove not to be durable enough for an application and the project team will entertain the idea of a semi- lubricious hydrophobic coating with higher durability. Apart from this consideration, there are some clear cases where one type of coating is required over another. For example, project teams will need to understand their
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device requirements with regard to water barriers and optics.
A primary purpose of hydrophobic coatings such as polytetrafluoroethylene or polyxylylene is to act as a barrier against water. If a device must be sealed so that moisture does not get inside, one of these hydrophobic coatings will work to repel water from the device’s surface and act as a sealant over areas where water can penetrate to the interior. Hydrophilic coatings do the opposite. They imbibe
water, and most of them are in fact comprised of more than 90% water when wet. Continuous diffusion of water across the thickness of these coatings occurs, and if this would be detrimental to the device, a hydrophilic coating is contraindicated. Granted, most medical hydrophilic
coatings rely on primer coats or base coats for adhesion to a surface, and these primers tend to be relatively hydrophobic, which could cause them to act as water barriers too. However, often they are not designed for these purposes and would not constitute as good a barrier as a material such as polytetrafluoroethylene or polyxylylene.
For devices that require lenses or other optics that interact with water, a hydrophobic coating could be a liability. Consider a newly waxed car with a hydrophobic finish. After a rainstorm, beads of water can be seen on the surface. On a camera lens or sensor in a medical instrument this beading can impair imaging. If these surfaces are instead coated with hydrophilic material, the opposite effect would occur: the water would spread evenly over the surface, producing a nominal anti-fogging effect.
Different business models There are crucial differences in business models for hydrophobic and hydrophilic coatings. Partially, these differences may be due to the maturity of the two industries, the hydrophobic coatings segment being much older. However, they also differ due to the level of know-how required, which stems from the physical realities of applying the coatings. Essentially, hydrophobic coatings have matured into
April 2011 ¦ 17
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